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Compulsive liars tell the stories they think want to be heard, while pathological liars "'continue to lie when they know you know they’re lying," says Dr. Ekman.

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Admit it: From time to time, you lie — at least a little. Your best friend asks what you think of her new haircut. It's awful, but you tell her it looks great. A spouse wants to know if that extra 10 pounds shows, and of course, you say it doesn’t.

"Lying is part and parcel of everyday life," says Robert Feldman, PhD, professor of psychological and brain sciences and deputy chancellor at the University of Massachusetts in Amherst. "In a sense, lies are the lubricants that move social interaction forward,'' says Dr. Feldman, who wrote The Liar in Your Life.

But when lying gets out of control it can wreak havoc in your personal and professional lives — potentially destroying relationships and careers.

Here's what you need to know about extreme lying.

Compulsive vs. Pathological Liars

Out-of-control lying is known as compulsive or pathological lying. Definitions are fluid, experts say.

Compulsive liars have a need to embellish and exaggerate, says Paul Ekman, PhD, a professor emeritus of psychology at the University of California in San Francisco and the author of Telling Lies, among other books. "They tell the stories they think want to be heard," he says. When you ask a compulsive liar for an opinion on an important issue, says Dr. Ekman, they’re likely to say something like this: "You know, you made a really wise choice in asking my opinion. Many people do. I've actually been asked by the governor of California to comment on this."

"Often, they’re pretty good liars," Ekman adds. "You often believe what they say — at least for a while."

Pathological liars may be even bolder. They ''continue to lie when they know you know they’re lying," Ekman says. The two lying types are pretty similar, he says, and actually, ''You could be a compulsive pathological liar."

Neither compulsive nor pathological lying has been studied extensively, say Feldman and Ekman. "I don't think we really know enough about the etiology [causes] of these to know if they should be considered a mental disorder," notes Ekman.

For example, experts don’t know for sure what drives the troublesome lying. They know impulsivity and a need to impress could be linked to the habit. But they’ve debated whether these types of lying are symptoms or a disease.

Liars' brains may differ structurally from the average brain. In a study in The British Journal of Psychiatry, scientists did brain scans on pathological liars and others, and found that the liars had more white matter in the brain's prefrontal cortex. They concluded that the increase in white matter may somehow provide these "super-liars" with ''the cognitive capacity to lie."

While everyday lies are goal-directed — you don't want to hurt the feelings of your overweight spouse — pathological lies often seem purposeless. Sometimes the lies are even self-incriminating, making them that much more difficult to figure out.

Compared to pathological liars, compulsive liars can get along pretty well in life, Ekman says. "Compulsive liars usually get away with it because they tell the lies we want to believe."

Fortunately, neither type of liar is common, according to Feldman and Ekman. Ekman estimates fewer than 5 percent of people lie compulsively or pathologically.

Can Compulsive or Pathological Liars Change?

In Ekman's experience, most liars who are compulsive or pathological don't want to change enough to enter treatment. Usually they only do so when directed by court order, after they've gotten into trouble, he says. Or they do so after their lies have resulted in dire consequences such as bankruptcy, divorce, or loss of a career.

Little research exists on treatment options for liars. Counseling or psychotherapy may help, with a focus on how to reduce impulsivity.

Spotting, Living With, or Working for a Whopper of a Liar

Can you tell on first meeting that someone might be a troubled liar? It’s difficult, but Ekman has found this rule-of-thumb helpful: "In the first half hour [of meeting someone], if I want to invite them home for dinner, I watch out!" he says. That means their charm, a characteristic of liars, may have worked its devilish magic.

If a new friend or acquaintance shows his colors as a compulsive or pathological liar, the mentally healthy thing to do is walk away, Ekman says. "What people value in friendships is truthfulness,” he says.

While those closely tied to a pathological liar may stay optimistic that the liar will change, Ekman tells them: "You also need to be a realist. Do you really want to spend your life, at work or at home, wondering if you’re being duped?"

Pathological liars are so good, Feldman agrees, ''so you won't know when you’re being lied to." Don't expect remorse, either, he says. "Pathological liars will look at a situation entirely from their own perspective. They have no regard for another's feelings about what might happen as a result of their lies," Feldman says.